Satellite-based positioning systems include constellations of earth orbiting satellites that constantly transmit orbit information and ranging signals to receivers. An example of a satellite-based positioning system is the Global Positioning System (GPS), which includes constellations of earth orbiting satellites, also referred to as GPS satellites. The GPS satellites transmit precise orbit information and ranging signals which are received by GPS receivers. The GPS receiver receives the satellite signal information from three or more GPS satellites and calculates its location by measuring the range of the receiver from the satellites and by determining the locations of the satellites. Location of a satellite is derived from ephemeris and almanac data transmitted by the satellite. The almanac data tells the GPS receiver where each GPS satellite of the constellation should be at any time over a wide time interval that spans a few days or weeks. The ephemeris data of each GPS satellite tells the GPS receiver with much more accuracy the specific location of the satellite over a much shorter period of time. For example, the ephemeris data of a GPS satellite predicts the satellite position, in addition to its velocity, clock bias, and clock drift over a future interval of approximately four hours. Therefore, correct demodulation and decoding of the ephemeris data are essential for determining the position of the receiver.
However, there are situations when a low signal strength of the satellite signals can prevent the correct demodulation or decoding of the ephemeris data. Low signal strength conditions can occur in challenging environments such as in urban canyons, under foliage, inside tunnels etc. Signal strength is typically expressed as a ratio of carrier power over noise power, or CN0, in units of dB-Hz. For example, conventional GPS data decode systems have not proven to be capable of operating reliably below a CN0 of 26 dB-Hz. In particular, the problem of undetected bit errors becomes severe when operating below the CN0 of 26 dB-Hz. GPS data bits are organized into GPS data word of 30 bits long. An undetected bit error occurs when a data decode system indicates a successful data word decode when in fact there are bits in the word that are wrong. Undetected bit errors in an ephemeris data decode can lead to large position errors. Under low signal strength conditions, the CN0 may drop below the minimum threshold of 26 dB-Hz required for conventional GPS data decode systems to operate reliably. Therefore, it is desirable to improve the sensitivity of ephemeris data decoding and to reduce the probability of undetected bit error in order to enable correct position determination of the GPS receiver when operating in low signal conditions.